Method for removing mercaptans from petroleum oil



Patented Mar. 2, 1943 METHOD FOR REMOVING MERCAPTAN S FROM PETROLEUM OIL Lawrence M. Henderson, Winnetka, and George W. Ayers, Jr., Chicago, lli., assignors to The Pure Oil Company, Chicago, 111., a corporation of Ohio No Drawing. Application March 14, 1941 Serial No. 883,340

15 Claims.

This invention relates to the removal of weakly acidic organic sulfur compounds from solution in otherwise substantially neutral organic liquids of the type of hydrocarbons and particularly is concerned with the removal of mercaptans from petroleum distillates.

It is frequently necessary or advantageous to remove small quantities of weakly acidic organic sulfur compounds, such as mercaptans and including thiophenols from their solutions in substantially neutral petroleum distillates. It has been common practice in the petroleum refining art to remove part of such sulfur compounds by washing with caustic alkali solutions and then to sweeten with doctor solution. A disadvantage in this type of treatment is that a considerable portion of the sulfur remains in the oil or distillate and adversely affects the anti-knock properties of the distillate. This is particularly undesirable where the distillate is to be used as a motor fuel. Motor fuel containing sulfur compounds, such as organic disulfides, not only has a lower octane rating than substantially sulfur-free motor fuel, but requires much more tetra-ethyl lead to raise the octane rating to a given value.

It is an object of this invention to provide a method whereby weakly acidic organic sulfur compounds of the type hereinbefore described can be eificiently removed from hydrocarbon type fluids.

It is another object of this invention to provide a method for removing mercaptans from low boiling petroleum distillates such as gasoline or kerosine to produce sweet or nearly sweet distillates.

It is a further object of this invention to provide a treating reagent useful in the removal of weakly acidic organic sulfur compounds from hydrocarbon type liquids.

Other objects and advantages will be apparent from the following detailed description.

It has now been discovered that alkaline solutions of salts of substituted or unsubstituted organic acids having at least one carboxyl group attached to an aliphatic carbon atom and hav- -ing one or more alml hydrogens replaced by halogen, are capable of reacting with weakly acidic organic sulfur compounds in solution in hydrocarbon type liquids, particularly petroleum chain, are preferred because of their greater water solubility. Although salts of the higher molecular weight acids are not inoperative, salts of these acids are in general of lower water solubility and have greater emulsifying tendencies and are, therefore, less desirable. How-, ever, it is apparent that the water solubility of a given acid may be materially altered by substituent groups. For example, the presence of hydroxy groups ordinarily increases the water solubility.

The alkaline solutions in which salts of the foregoing acids may be used include aqueous or alcoholic solutions of alkali metal hydroxides,

ammonium hydroxide, quaternary ammonium bases, sulfonium hydroxides and in general all strongly alkaline bases. The alkali metal hydroxides are particularly eflective and are the strongest of the readily available bases. Sodium hydroxide is generally preferred because of its lower cost.

The metal which may be used to form the salt of the aforementioned organic acids may be any metal including ammonium which produces salts that are appreciably soluble in the alkaline solutions. In general, the alkali metal salts, particularly the sodium salts, are preferred.

The removal of weakly acidic organic sulfur compounds such as mercaptans from petroleum distillates by treatment with alkaline solutions of salts in accordance with this invention, should not be confused with methods of treating such distillates with strong alkaline solutions and containing salts of various organic acids as solubility promoters or solutizers. In the latter method of treatment the salt of the organic acid is employed to enhance the extraction power of aqueous alkali solutions for the organic acidic compounds. In the method contemplated by this invention, the removal of the mercaptans is not dependent upon the extraction power of aqueous alkali solution and in-- stead relies upon chemical reactions involving,

the organic acidic compounds, alkaline material and the salts of the halogen substituted organic acids. It is believed that this reaction takes place in accordance with the following equations using a mecaptan as the organic acidic compound, sodium hydroxide as the alkaline material and sodium chloracetate as the salt of a halogen substituted organic acid:

(2) RSNa+ClCHa.COONa- RSCHmCOONa-i-NaCl In accordance with the foregoing equations, it

sulfur compounds which it is desired to remove,

up to concentrated alkaline solutions containing about 40% by weight of alkali. However, in general, it is preferred to employ relatively dilute alkaline solutions, that is, solutions which contain only a slight excess, up to about 5% by weight in excess of the amount of alkali required for reaction with the weakly acidic organic sulfur compounds. This amount of alkali may be readily determined in the case of mercaptans in accordance with Equation 1.

The amount of salts of the aforementioned halogenated acids which it is desired to use may vary between wide limits depending upon the amount of acidic organic compounds which it is desired to remove. When it is desired that the treated liquid be sweet to the: doctor test, it is preferred to use at least an amount of salt stoichiometrically equivalent to the alkaline material molecular proportions of alkali and salt of halogen substituted acid in order that both materials may be consumed substantially completely thereby avoiding the waste incurred by having a solution exhausted as to one component but not as to the other.

Since this reaction is not dependent upon the extraction power of the alkali which, as is well known, diminishes with increase in temperature, the reaction may be expedited by using elevated temperatures, such as temperatures upwards of 90 F. However, this reaction will also proceed at normal temperatures and it is, therefore, unnecessary to supply additional heat, unless the reduction of reaction time thereby achieved makes it advisable to do so. Low treating temperatures of the order of 30 F. to 40 F. are generally to be avoided due to the slower reaction and to greater difllculty in effecting separation of the treating reagent from the material treated.

'After treating the distillates with alkaline treating reagents, in accordance with this invention the treated distillate and spent treating Removal of mercaptans from gasoline DA-44 by treatment with sodium monochloracetate in NaOH solution Per cent RSH- sulfur removed Grams QICHQCOOH per 200 co. gasoline In each of the foregoing tests 200 cc. samples of gasoline were shaken with the indicated amounts of monochloracetic acid and the samples containing the monochloracetic acid then shaken with 10% by volume of 5% by weight aqueous sodium hydroxide solution. After allowing a few minutes for the separation of a gasoline layer and a treating reagent layer, the gasoline was separated and washed with an equal volume of water.

The results in Table I clearly show that the .mercaptan sulfur content of gasoline may be materially reduced by treatment with dilute sodium chloracetate solution. While in the particular examples shown chloracetic acid was added to the gasoline first and the sodium salt formed in situ, satisfactory results could also have been obtained by employing an alkaline solution of sodium chloracetate by first mixing sodium hydroxide solution with the gasoline and subsequently adding the monochloracetic acid or by employing separate solutions of sodium hydroxide and sodium chloracetate and mixing these solutions simultaneously with contact with the gasoline. Generally speaking, it is preferable to form the salt in situ or to follow any other suitable procedure which avoids havin the salt in contact with alkaline solution prior to contact with the fluid to be treated inasmuch as more efllcient use of the reagents is thereby obtained. This is due to the tendency of the halogen to hydrolyze in alkaline solutions forming the corresponding hydroxy compound. This hydrolysis may be substantially avoided by maintaining the alkaline solution of the salt at low temperatures, for example about 40 F. However, the entire difilculty with hydrolysis may be avoided by avoiding an excess of alkaline material in the salt solution until the components are in the presence of thematerial to be treated or forming the salt in situ. In general, the latter method is preferred.

In order to conserve the treating solution as much as possible and to confine the reaction of the treating solution to weakly acidic organic sulfur compounds that are not readily removed by other methods, it is preferred to pretreat the distillates in a suitable manner to remove the stronger acidic compounds, such as hydrogen' sulfide by washing with water or scrubbing with a dilute caustic solution and thereafter subjecting the pretreated distillate to the treatment hereinbefore described.-

The specific examples given are only by way of illustration and are not intended to limit the invention to the specific quantities given or the particular compounds named. Mixtures of various salts of organic acids within the scope of this invention, as well as mixtures of various alkalis may be employed. It i important, however, that suificient alkali be used to react with the weakly acidic organic sulfur compounds in order to obtain the full benefits of the invention. It will be apparent to those skilled in the art that various expedients may be resorted to without departing from the spirit of the inven- -tion. The invention is, therefore, to be interpreted as broadly as the prior art permit in view of the following claims.

We claim:

In a method for removing weakly acidic organic sulfur compounds from otherwise substantially neutral hydrocarbon fluids the step which comprises contacting said fluids with aqueous alkaline solution containing a hydrolyzable salt of an unsubstituted organic acid having at least one carboxyl group attached to aliphatic carbon but having one or more alkyl hydrogens replaced by halogen.

2. Method in accordance with claim 1 where the organic acid is a fatty acid having not over about ten carbon atoms in the aliphatic chain.

3. Method in accordance with claim 1 Where the alkaline solution is alkali metal hydroxide solution.

4. Method in accordance with claim 1 where the halogen is chlorine.

5. Method in accordance with claim 1 where the salt of the organic acid is alkali metal salt.

6. In a method for removing mercaptans from petroleum oils, the step which comprises con tacting said oils with aqueous solution containing alkali metal hydroxide and a hydrolyzable alkali metal salt of an unsubstituted organic acid having at least one carboxyl group attached to alithe alkali metal hydroxide solution is a sodium hydroxide solution and the organic acid salt is a sodium salt.

11. Method in accordance with claim 6 where the organic acid salt is sodium chloracetate.

12. Method in accordance with claim 6 where the alkali metal hydroxide solution is a dilute solution and the amount of organic acid salt is approximately molecularly equivalent to the amount of alkali metal hydroxide.

13. In a method for removing mercaptans from petroleum distillates. the step which comprises reacting said distillates with aqueous sodium hydroxide solution containing sodium chloracetate.

14. Method in accordance with claim 13 in which the sodium chloracetate is formed from chloracetic acid in the presence of the petroleum distillates.

15. Method in accordance with claim 13 where.

the sodium hydroxide solution is a dilute solution and the amount of sodium chloracetate is approximately molecularly equivalent to the amount of sodium hydroxide.

LAWRENCE M. HENDERSON. GEORGE W. AYERS, JR. 

